BJB
Joined: Jul 28, 2008
Posts: 413
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Posted on Sep 12 2013 05:59 PM
Yeah, I pretty much expected that to happen. Is it possible to re-drill the holes and shift the board slightly to the left? If you look at the picture of my reverb unit, there is room under the pilot light for components. You don't need to move the board that far, just enough to spare the power supply capacitors from unnecessary heat.
Ideally, though, the board layout would have those blue capacitors on the extreme left and the reverb driver tube cathode resistor and capacitor right above the socket. Once again, take a look at my layout and you will see that the cathode resistor and capacitor is right next to the tube socket. It's almost as though someone planned it that way!
— If it ain't broke, fix it until it is.
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crumble
Joined: Sep 09, 2008
Posts: 3158
Guildford England
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Posted on Sep 12 2013 09:18 PM
I appreciate your company on this BJB but I wouldn't be too concerned with board position because it will only be used occasionally for recording.
It'll be a few weeks before I can get started on it again because i'm having problems finding a BR310 bridge rectifier.
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BJB
Joined: Jul 28, 2008
Posts: 413
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Posted on Sep 13 2013 01:52 PM
That's OK. This temperature thing has been bugging because I'd like to know for sure if this truly is a problem. In real life, I'm an electrical engineering person and these sorts of issues come up every so often. There always the 'right' way to do things, but sometimes we can't do things 'right' and the question becomes: Can I get away with doing this the wrong way? If I can justify doing something the 'wrong' way and I save a lot of trouble, then I've earned my pay.
Yesterday I remembered that one of my digital meters came with a temperature probe attachment that I've never bothered to use. So I loosened the back cover of my reverb unit, inserted the probe near the driver tube, and tightened the cover back down again. Measuring and recording the ambient and temperature probe reading, I found a maximum temperature of 35C/95F at an ambient temp of 26C/79F. Even if my reading was artificially low, a temperature rise of 10C or so isn't anything to worry about. This environment is well within the design limits of the capacitors so there is no concern about lifetime reduction caused by their location relative to the reverb driver tube.
— If it ain't broke, fix it until it is.
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cambeezy
Joined: Jun 26, 2007
Posts: 399
Cleveland, Ohio
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Posted on Sep 13 2013 08:27 PM
Anyone familiar with Fender Blues Jr amps knows how hot those EL84 power tubes get. Years ago my son left the amp on overnight -- those tubes were sizzlin hot. Miraculously, nothing melted inside and it still works.
So I'm guessing those caps are pretty rugged as my tanks don't get nowhere near as hot as a BJr.
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BJB
Joined: Jul 28, 2008
Posts: 413
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Posted on Sep 14 2013 12:51 AM
Now that I know how to measure temperatures, it might be fun to see what kind of mayhem is going on inside of Fender amps. The Pro Junior also has a reputation for running hot and I've seen one that had a board that was burned where the power tubes attach. On another guitar forum, a tech recommended lowering the bias so the amp would run a little cooler, which was probably a pretty good idea. Clip-on fans are nice to use on any amp, and I was pleased to see them in use at the recent SG101 convention.
Fender uses Illinois capacitors, which tend to be looked down upon among in the guitar forums. But they are rated at 85C, unlike those Sprague Atoms on that Hoffman board are only good for 65C. If you look at the hi-fi tube forums, anything by Sprague (Orange Drops and Atoms) are openly mocked. Those guys prefer the Mallory or F&T caps. Of course, the Mallory 20uF/450V caps cost about $25 each, so they'd better be good!
— If it ain't broke, fix it until it is.
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crumble
Joined: Sep 09, 2008
Posts: 3158
Guildford England
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Posted on Sep 14 2013 08:57 AM
It's fair comment. An amateur such as myself would soon find himself out of his depth arguing with a qualified engineer. My own gut feeling is that the components on my board are adequate for the job and at this point I shouldn't be changing components unless there is a problem.
My '79 Twin Reverb's original Mallory filter caps were still in good working condition in 2012! they were changed as precautionary due a slight noise when cold.
Just out of interest what is you opinion of old style paper caps.
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Noel
Joined: Mar 15, 2011
Posts: 8528
Back in Piitsburgh, Pennsylvania, where I grew up.
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Posted on Sep 14 2013 09:49 AM
This is why I studied Primate Paleontology in college. Also, I never use old-style paper clips to replace fuses. 
— This is Noel. Reverb's at maximum an' I'm givin' 'er all she's got.
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crumble
Joined: Sep 09, 2008
Posts: 3158
Guildford England
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Posted on Sep 14 2013 10:03 AM
Noel wrote:
This is why I studied Primate Paleontology in college. Also, I never use old-style paper clips to replace fuses.
Appreciated !
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